The present invention relates to a fluid diffusion electrode of a solid polymer electrolyte fuel cell, and in particular to a method of abrading a surface of a fluid diffusion layer of the electrode and a product to which the method has been applied.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d), which comprises an ion exchange membrane, or solid polymer electrolyte disposed between two fluid diffusion electrodes typically comprising a layer of porous, electrically conductive substrate material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit.
At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode catalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode catalyst layer to form a reaction product. In fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode catalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H2xe2x86x922H++2exe2x88x92
Cathode reaction: xc2xdO2+2H++2exe2x88x92xe2x86x92H2O 
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell.
Two or more fuel cells can be electrically connected together in series to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field or separator plate can serve as an anode plate for one cell and the other side of the fluid flow field or separator plate can serve as the cathode plate for the adjacent cell. Such a multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates.
Conventional fuel cell electrode designs typically comprise a fluid diffusion layer (FDL) and a catalyst layer. The FDL generally comprises an essentially fluid-permeable substrate, and in some cases, a sublayer disposed on one surface of the substrate for providing a base on which a catalyst layer is disposed to form an electrode. The substrate serves as a backing material and structural support for the electrode, and is typically made of an electrically conductive material such as carbon cloth, carbon paper, carbon fiber woven, or carbon fiber non-woven. A hydrophobic polymer such as polytetrafluoro-ethylene (PTFE) is typically applied to the substrate to discourage water (either generated from the electrochemical reaction or from the humidified reactant streams) from accumulating in the electrode. The PTFE-treated substrate is typically sintered so that the hydrophobic polymer melts and coats the substrate.
The sublayer, if present in the FDL, is generally concentrated at the catalyst side of the substrate. The sublayer generally comprises fibers or particles of an electrically conductive material such as carbon or graphite, and may also contain some hydrophobic material such as PTFE. Several types of high surface area carbon particles, both graphitized and non-graphitized, are available for use in the sublayer. The catalyst is typically applied to the substrate surface coated with the sublayer (although such a fluid diffusion layer could be combined with a catalyzed membrane in an MEA). Suitable catalyst materials include precious metals or noble metals such as platinum. The catalyst layer may comprise unsupported catalyst such as platinum black, or include supported catalyst in which catalyst such as platinum is supported on for example, carbon particles.
There is motivation in the fuel cell industry to improve long-term performance and reliability of MEAs while reducing their manufacturing costs. Low cost materials and simplified processing steps are desirable, but the MEA should meet minimum standards of reliability, longevity and performance. For example, the MEA materials should be selected and the MEA manufactured such that the MEA maintains membrane integrity over its designed operating life. Membrane integrity is necessary to maintain fluid isolation of the fuel and oxidant streams during fuel cell operation; a perforation in the membrane can cause reactant transfer leaks (that is, a leakage of one or more reactant through the membrane to the other electrode) which can be detrimental to fuel cell performance and can further damage the cell. Various approaches have been developed to detect membrane perforations and associated reactant transfer leaks; one such approach is described in U.S. Pat. No. 5,763,765, owned by the Ballard Power Systems Inc., the assignee of the present application. In the approach described in the ""765 patent, perforations in a membrane are detected by a thermal imaging device that detects heat generated by an exothermic reaction of a pair of reactants which contact each other at a membrane perforation. The localized exothermic reaction appears as a xe2x80x9chotspotxe2x80x9d in the thermal image.
A correlation has been identified between certain surface texture characteristics of the FDLs of an MEA in a solid polymer electrolyte fuel cell and the occurrence of membrane perforations and transfer leaks in operating fuel cells. Examples of such surface texture characteristics include xe2x80x9csurface roughnessxe2x80x9d and xe2x80x9cwavinessxe2x80x9d; in the context of this description, surface roughness relates to the finest (shortest wavelength) irregularities of a surface and waviness relates to the more widely spaced (longer wavelength) deviations of a surface from its nominal (intended) shape that cause the profile of the electrode or FDL of the electrode to vary in thickness.
In one embodiment, a method of manufacturing an FDL for a solid polymer electrolyte fuel cell comprises abrading a surface of the FDL such that the topography of the FDL surface is rendered more uniform, leading to reduced surface roughness and/or waviness. The FDL comprises at least a porous substrate and may also comprise a carbon-containing sublayer on the surface of the substrate. The sublayer provides a support layer for the deposit of catalyst on the substrate. The FDL may also comprise a hydrophobic material such as polytetrafluoroethylene (PTFE).
In the manufacture of such an FDL that does not already comprise hydrophobic material, a hydrophobic material such as PTFE may be applied to the substrate before or after the substrate is abraded. After the hydrophobic material is applied, the substrate is sintered (before or after abrading) so that the hydrophobic material melts and coats on the substrate, thereby rendering the FDL more hydrophobic. If the FDL does not already have a carbon-containing sublayer, a sublayer may be applied on the substrate before or after sintering, before or after abrading, and before or after the application of the hydrophobic material. A final sintering step may be carried out after the sublayer (and optionally, the hydrophobic material) has been applied.
The sublayer may contain, in addition to carbon, a percentage of hydrophobic material. If such a hydrophobic sublayer is applied to the substrate, a sintering treatment may be carried out to melt the hydrophobic material in the sublayer and distribute it over the substrate. Abrading the sublayer-coated substrate has been found to lead to an improvement in the uniformity of the sublayer thickness and the FDL or substrate basis weight (density), and a reduction in the number of significant protrusions on the substrate that may cause damage to an adjacent membrane.
The FDL surface is abraded with an abrading material having an average Ra (average surface roughness) that is less than the average Ra of the FDL prior to abrading. The abrading treatment has significant benefit for FDLs having an average Ra of at least about 14 xcexcm, and that are abraded to an average Ra of between about 6 xcexcm and 10 xcexcm. Ra is a standard surface profile parameter used in the surface finishing industry (see for example, the Surface Metrology Guide developed by Precision Instruments Inc., ASME B46.1-1995, ASME B46.1-1985, ISO 4287-1997, and ISO 4287/1-1984) and is defined as the area between the roughness profile of a surface and its mean line, or the integral of the absolute value of the roughness profile height over the evaluation length. Note that Ra measures the profile of a section of a surface (i.e. is a two-dimensional measurement); in contrast, the xe2x80x9caverage Raxe2x80x9d relates to the three-dimensional topography of a surface and is the averaged value of a plurality of sectioned profiles of the surface. While Ra values are one way to quantify the effects of abrading, other measurable industry standard surface profile parameters such as peak count (Pc) or Kurtosis (Rku) may be employed.
An abrading treatment can also be beneficial to reduce the waviness of an FDL. Waviness can be measured by measuring the standard thickness deviation over a sectioned profile of an FDL material (and the corresponding average standard deviation over a selected surface area). The abrading treatment has significant benefit for FDLs having an average standard deviation thickness of about 28 xcexcm, and that are abraded to an average standard deviation thickness of less than about 15 xcexcm.
An abrading treatment is particularly effective for FDLs comprising or consisting essentially of porous substrates such as carbon fiber woven or non-woven. Such substrates tend to have a pore volume of at least about 80% and an average pore size of at least about 30 xcexcm. For carbon fiber non-woven substrates, abrading with 320 grit sanding material has been found to be effective in reducing the average Ra (or Pc) to desirable levels.
In another embodiment, an FDL for a solid polymer fuel cell has a surface abraded to an average Ra of between 6 and 10 xcexcm. The FDL comprises a substrate that may be a significantly porous substrate, having a substrate pore volume of at least about 80%, and an average pore size of at least about 30 xcexcm. The substrate may be made from carbon fiber woven or non-woven material. The FDL may further comprise a carbon-containing sublayer on a surface of the substrate. An electrode may be formed by coating a catalyst on the FDL; the catalyst may be a carbon-supported catalyst. Two such electrodes interpose a solid polymer electrolyte membrane to form a membrane electrode assembly (MEA). The MEA is interposed between a pair of fluid flow plates to form a fuel cell. The fuel cell may be combined (typically in electrical series) with other fuel cells to form a fuel cell stack.
In the above embodiments, substrate and sublayer particles that are loosened as a result of the abrading operation are typically removed in a cleaning step prior to applying the catalyst coating.
In another embodiment, a sublayer-free FDL is abraded and the loosened particles are deposited into pores of the substrate such that they form part of the FDL. xe2x80x9cDepositxe2x80x9d includes allowing the substrate particles to fall into the pores. The substrate may comprise hydrophobic material on its surface and in its pores; if not, a hydrophobic material may be applied to the substrate prior to abrading. The loosened particles would thus comprise abraded substrate particles as well as some abraded hydrophobic material. The hydrophobic material, either abraded as a result of abrading or still in place on the surface or in the pores of the substrate, secures the abraded substrate particles to the substrate.
The abraded substrate particles that are deposited onto the substrate preferably primarily occupy the pores of the substrate near the substrate surface (xe2x80x9csurface poresxe2x80x9d). The substrate may then be subjected to a post-abrading sintering step (a pre-abrading sintering step may also be carried out after the hydrophobic material is applied to the substrate). Additional hydrophobic material may optionally be applied to the substrate after abrading and before the post-abrading sintering step. The xe2x80x9cfillxe2x80x9d of loosened substrate particles provides a base for the catalyst layer, thereby in most cases obviating the need to apply a separate carbon containing sublayer to the substrate, thereby simplifying the FDL manufacturing process. The abrading operation also improves the uniformity of the surface topography of the substrate.
In another embodiment, an FDL for a solid polymer electrolyte fuel cell comprises a hydrophobic porous abraded substrate with deposits of abraded substrate particles in at least some of the pores of the substrate. A carbon sublayer may be applied between the substrate and the catalyst sublayer, but is generally not necessary if the loosened substrate particles occupy the surface pores of the substrate so as to provide an adequate base for the application of the catalyst. The FDL may also comprise or consist of significantly porous substrates such as carbon fiber woven or non-woven. Such substrates have a pore volume of at least about 80% and an average pore size of at least about 30 xcexcm.
A catalyst coating may be added to a surface of this FDL to form an electrode. A pair of such electrodes interpose a solid polymer electrolyte membrane to form an MEA. The MEA is interposed between a pair of fluid flow plates to form a fuel cell. The fuel cell may be combined with other fuel cells to form a fuel cell stack.